1. Field of the Invention
The present invention relates to an improvement of a PWM (pulse width modulation) power converter with high power conversion efficiency such as an inverter, a converter, a forward-reverse power converter, an active filter, and the like, which is arranged such that a plurality of bridge-connected switching elements are driven based on a PWM signal.
2. Description of the Related Art
As is known, a plurality of bridge-connected switching elements are driven based on a PWM signal, and a PWM power converter with high power conversion efficiency, such as an inverter, a converter, a forward-reverse power converter, an active filter, and the like can be constituted.
The switching elements used in the PWM power converter include relatively low-speed power self-extinction type elements such as BPTs (bipolar transistors), GTOs (gate turn-off thyristors), and the like. The modulation frequency for PWM falls within the range of 500 Hz to 2 kHz, and a PWM pulse count falls within the range of several pulses to pulses between 10 and 20. The modulation frequency for PWM falls within the range of 500 Hz to 2 kHz, and the PWM pulse count consists of a combination of relatively long pulses, e.g., several pulses to pulses between 10 and 20.
However, in a PWM waveform consisting of a pulse train of long pulses, in order to maintain an input or output signal waveform of a power converter to be a sine wave, a filter using an LC resonance circuit consisting of coil L having a large reactance and capacitor C having a large capacitance is necessary.
In particular, an active filter for a PWM waveform consisting of a pulse train of long pulses between 10 and 20 can only function up to harmonics of lower orders. Thus, a demand has arisen for an active filter which can function up to harmonics of higher orders.
Recently, high-speed power switching elements such as SIT (static induction transistors), SI thyristors (static induction thyristors), and the like are available. For this reason, a modulation frequency for PWM can be set to be several tens of kHz, and a PWM pulse count can be obtained by combining several hundreds of short pulses.
Since the modulation frequency for PWM is increased, an input or output signal waveform of a power converter can be a sine wave free from distortion, and an active filter can function up to harmonics of higher orders.
However, in a high-frequency PWM power converter with an increased modulation frequency for PWM, the following problem is posed. As the modulation frequency is increased, a switching loss of each switching element is increased, and power conversion efficiency is reduced. More specifically, a self-extinction type power high-speed switching element has a trade-off relation between its switching time and ON voltage. For this reason, in a conventional high-frequency PWM power converter, elements capable of high-speed operation are used as switching elements at the cost of ON resistances of switching elements which are used for increasing a modulation frequency.
As a result, the modulation frequency of the PWM power converter is increased, while power conversion efficiency is degraded due to high ON resistances of switching elements used. Thus, high power conversion efficiency is not obtained. In other words, use of high-speed switching elements allows a decrease in switching loss but causes an increase in conduction loss. Therefore, conversion efficiency as a whole is limited.
Note that semiconductor switching elements include MOS (metal oxide semiconductor) transistors, IGBTs, and the like in addition to the BPTs, GTOs, SITs, SI thyristors and the like described above.
A dead time is set between adjacent high-frequency PWM signals supplied to bridge-connected switching elements in order to prevent the elements series-connected to a DC power supply from being simultaneously turned on. A rate of a decrease in pulse width (lean of a pulse width) of the PWM signal due to the dead time largely affects an output signal waveform more than a case of a PWM signal consisting of a pulse train of long pulses. Thus, fidelity of an output signal waveform with respect to an input signal waveform is impaired.
When a high-frequency PWM signal with an increased modulation frequency is used, pulse dropout occurs in a crest region and a zero-crossing region of an output signal waveform. Thus, the fidelity of the output signal waveform with respect to the input signal waveform is also impaired in the crest region and the zero-crossing region of the output signal waveform.